When I heard Showtime would be running a series called “Masters of Sex,” I smirked. Along with its roster of well-known quality shows, including “Nurse Jackie” and “Homeland,” the channel has always had a few titillating soft-core series hiding in its back room, such as “Red Shoe Diaries” in the 1990s and the current series “Gigolos.”

But “Masters of Sex” is far from a piece of cable erotica. About the pioneering sex researchers William Masters and Virginia Johnson, it’s an inviting, beautifully acted, and smartly written period drama set in the 1950s. It contains plenty of sexual content, when the pair monitor their subjects — most of them prostitutes and johns — during sexual activity; but it isn’t sexed-up so much as it is about sexuality. It’s also about science, primitive then compared with what it is now, and the cultural resistance at the time to talking openly about sex.

The show, which premieres Sunday at 10 after the return of “Homeland,” is a classic example of a series that starts off slowly and creakily, with lots of superficial setup, before it takes off in episodes two and three. As played by Michael Sheen, Masters is a cranky, cool doctor of obstetrics and gynecology at Washington University teaching hospital. He’s celebrated for his fertility work, but he is fixated on going beyond reproduction to explore the realities of sexuality. The hospital, headed up by Beau Bridges’s Barton Scully, won’t allow him to conduct the study on campus, so he takes to paying to watch from a closet in a nearby bordello. His vacant marriage to the submissive Libby (Caitlin FitzGerald) is marked by infertility, which leaves her feeling inadequate.

Johnson, played by Lizzy Caplan, enters Masters’s life as a secretary and assistant, but he quickly learns that her warm way with the subjects is essential to his success. She brings the personal touch that eludes him. A confident woman who enjoys sexual relationships without commitment, she isn’t even slightly embarrassed as they watch the subjects’ sexual activity together. She is a necessary partner, a fact that he ultimately has to admit despite the bruise to his ego. She’s having an affair with one of Masters’s residents, Ethan Haas (Nicholas D’Agosto), who is more emotionally needy than she’d like.

CRAIG BLANKENHORN/SHOWTIME

Caitlin Fitzgerald plays Masters’s wife, Libby.

All this is established somewhat dryly. But soon, the show’s deeper themes emerge, largely through the performances. Sheen is tense and more and more compelling with each episode, a complex and broken man hiding behind a hard face and an office filled with prizes. As his flaws and the reasons for his interests emerge, the facets of his performance become more visible. Early on, we learn that Libby can’t get pregnant because Masters has a low sperm count, but Masters doesn’t tell her that. Instead, he continues to press her into hospital procedures that seem doomed to fail. It’s an abusive streak that Sheen plays quietly, but without ambiguity.

MASTERS OF SEX

Why does such a clinical man, who makes passionless love to his wife, want to scrutinize sexuality and pry it out of taboo? That’s one of the running mysteries of “Masters of Sex,” which is adapted from the biography by Thomas Maier. Caplan is strong, too, as the woman in Masters’s life who may have mastered him. Her diction is overly precise, which sometimes feels like the actress’s way of saying, “This is the 1950s, when people spoke more formally.” But her performance grew on me, and there’s no denying the natural chemistry — not sexual, but intellectual — between her and Sheen.

This is a fall season filled with network drama premieres that offer little more than thrill-a-minute plots involving criminals, cops, serial killers, superheroes, and vampires. “Masters of Sex” is the one new series with any true subtlety, with any thematic range and depth. Unlike the others, it has a human touch.